The present invention relates to the construction of a semiconductor device in which a passivation layer is formed over an electrode.
In a semiconductor device in which a semiconductor element such as a diode, transistor and the like is formed on a semiconductor substrate, and an electrode (wire) is formed on the top of this, a passivation layer that includes an insulator is formed over most of the entire surface other than portions where the bonding wire of an electrode (bonding pad) is formed. By forming this kind of passivation layer, the semiconductor element is protected, and thus it is possible to stabilize the operation of the semiconductor element, and increase the reliability of the semiconductor device over a long period of time. This kind of passivation layer is particularly important in a power semiconductor device in which high voltage is applied and a large current flows.
A passivation layer, together with having high insulation property, requires having high humidity resistance and must be able to block impurities (ions) from having an adverse effect on the operation. Moreover, a passivation layer must also be able to perform sufficient mechanical protection of the semiconductor element. In order to obtain high humidity resistance and the ability to block impurities, being able to form a film on a semiconductor substrate as an insulating film that is dense is required, and as such material, silicon nitride film (Si3N4) is known. When a semiconductor substrate is made using a silicon-based material (Si, SiC), the elements of the silicon nitride film do not have an adverse effect on the operation of the semiconductor element, so using silicon nitride film is particularly preferred.
However, in order to obtain a high insulating property and to mechanically protect the semiconductor element, it is required that the passivation layer be made sufficiently thick. However, from the aspect of internal stress and adhesion of a typical silicon nitride film, peeling occurs easily, and forming a thick film is difficult so, forming a passivation layer made of only silicon nitride film is extremely difficult. Therefore, the construction described in patent literature 1 and 2 describes forming a silicon oxide film (SiO2) that has less ability as a barrier than silicon nitride film because silicon oxide film is not dense, however can be formed to be thick and also has high insulating characteristics, so this silicon oxide film is formed to be thicker than the silicon nitride film and is layered on the silicon nitride film. In this case, the silicon nitride film and silicon oxide film can both be formed by a PCVD (plasma CVD) method or the like. Moreover, adhesion between the silicon oxide film and silicon nitride film is high, so it is difficult for peeling to occur between these films. Furthermore, an organic film that is formed on the very top surface of the passivation layer made of a polymer material such as a polyimide or the like is formed by coating or the like so as to be sufficiently thick. Adhesion between the organic film and the silicon oxide film is also high, so it is also difficult for peeling to occur between these films.
Moreover, when the semiconductor material of the semiconductor substrate is, for example, n-type silicon carbide (SiC), and the semiconductor element that is formed is a Schottky barrier diode, the Schottky electrode that comes in direct contact with the semiconductor material is made of titanium (Ti), molybdenum (Mo) or the like. These metal materials are effective for forming a Schottky barrier, however, the electrical resistance is not low, or the surface oxidizes easily, so these metals are not preferable as an electrode or wire material for electrical connection, or as a material for a bonding pad for connecting a bonding wire or the like. Therefore, as the electrode structure that is formed on top of a Schottky barrier diode, a metal layer that includes aluminum (A), gold (Au), copper (Cu) or the like that has low electrical resistance and is more suitable for use as a wire material is thickly formed, and as a result it is possible to obtain a Schottky barrier diode that has small parasitic resistance.
A passivation layer such as described above is formed so as to cover mostly an entire semiconductor substrate on which semiconductor elements are formed, and parts of a metal layer such as described above of an electrode is formed so as to be locally exposed from the passivation layer. This exposed portion of metal layer is used as a bonding pad for connecting to a bonding wire or the like. By using this kind of construction, it is possible to increase the reliability of a power semiconductor device. When doing this, in order to sufficiently prevent moisture and impurities from entering into the semiconductor side from the thick silicon oxide film or organic film side, layered construction such as described above in which the silicon nitride film is provided further on the bottom side (side near the semiconductor elements) is preferred.